Resonant-tunneling diode on the basis of silicon multilayer cathode

The quantum mechanical phenomenon of tunneling is of interest from fundamental as well as device points of views. Semiconductor microstructures, and in particular asymmetric double quantum well structures (a-DQWS), provide ideal systems for investigating tunneling phenomena, especially when an electric field is applied to bring various electronic levels into resonance. We first discuss various resonant and nonresonant tunneling processes for electrons and holes, including both coherent and incoherent resonant tunneling and then review recent extensive investigations of resonant and non-resonant tunneling rates of electrons and holes in asymmetric double quantum well structures using time resolved spectroscopy. We also show that cw and ultrashort pulse experiments probe different physics and that it is difficult to obtain quantitative information about resonances and resonant tunneling rates from cw experiments.

Resonant tunneling(RT) phenomenon in double-barrier(DB) heterostructure[1,2] has a conceptual similarity to a transmission of optical waves in Fabry-Perot (FP) resonator and involves time delay. Its dynamics should be investigated since they limit the ultimate speed of RT devices. Such a study will also clarify similarities and differences between electronic and optical waves. In our previous work[3], we investigated the tunneling escape process of electrons from AlAs/GaAs/AlAs DBRT structures. The measured escape rate was well explained by the idealized theory of FP-like model, which predicts the tunneling escape time τ in DBRT structures is given by |t| 2vk/Lw, when |t|2 ≪1, where t is the transmission coefficient through the barrier, vk is the group velocity of electrons and Lw is the well width. This predicted escape time is equal to the one calculated by the sequential tunneling model, suggesting that the tunneling escape time is not strongly dependent on the coherency of electron waves. This simple relation may not hold for the tunneling process between quantum wells(QW), where resonant coupling effect plays a more sophisticated role. To clarify the resonant tunneling phenomena between QWs, we report, in this paper, our study on electron dynamics in several different double GaAs QW structures separated by thin AlAs barrier, where the coupling condition between QWs was varied by electric fields. Tunneling process was studied at ~20K by measuring time resolved photoluminescence(PL). Picosecond pulses of a mode-locked dye laser were used to generate electron hole pairs in QWs, and the subsequent PL from particular QWs was monitored by a streak camera to determine the time variation of electron density in the QWs. Note that the electrons are lost either by recombination (radiative[4] and nonradiative) and/or by tunneling process. Since the mass of heavy hole is quite heavy, hole tunneling can be neglected at least in the initial phase of tunneling.

A new type of field emission resonant tunneling diode has been proposed and investigated both theoretically and experimentally. The diode is based on an Si–SiOx–Si multilayer cathode containing an SiOx layer as the input potential barrier, an Si layer as the quantum well, and a vacuum layer as the output potential barrier of a double barrier quantum structure. The calculation predicted the existence of the resonant maxima (three or four depending on the input barrier height) of the current density–electric field dependencies. Frequency dependencies of the diode microwave impedance pointed to the existence of negative conductance resulting from resonant tunneling through an energy level in the quantum well (QW) at electron transit angle values in the interval from zero up to near 2π/3. Also found is a peak of negative conductance on the frequency dependence with the greatest peak value ∼57 S/cm at a frequency of 0.63 THz. The maximum upper frequency of the negative conductance band of more than 2 THz at a transit angle near 0.45π is reached when resonant tunneling occurs through the third resonant level in the QW. Experimental results confirmed the existence of the resonance peak in the investigated resonant-tunneling structure.

Tunnel nanojunctions based on inelastic electron tunneling (IET) have been heralded as a breakthrough for ultra-fast integrated light sources. However, the majority of electrons tend to tunnel through a junction elastically, resulting in weak photon-emission power and limited efficiency, which have hindered their practical applications to date. Resonant tunneling has been proposed as a way to alleviate this limitation, but photon-emissions under resonant tunneling conditions have remained unsatisfactory for practical IET-based light sources due to the inherent contradiction between high photon-emission efficiency and power. In this work, we introduce a novel approach that leverages much stronger resonant tunneling enhancement achieved by multiple metallic quantum wells, which has enabled the internal quantum efficiency to reach ∼1 and photon-emission power to reach ∼0.8 µW/µm2. Furthermore, this method is applicable with different electronic lifetimes ranging from 10 fs to 100 fs simultaneously, bringing practical implementation of IET-based sources one step closer to reality.

We present a self-consistent analytical model describing transport processes in quantum well infrared photodetectors (QWIPs) under dark conditions. The model takes into account electron thermionic emission from the quantum wells, thermionic injection from the emitter contact, and features of transport and capture in the self-consistent electric field in the QWIP active region. Using an assumption that the rates of the electron escape from and capture into a QW are functions of the electric fields only in the barriers sandwiching this QW, we calculate the electric field and charge distributions as well as dark current–voltage characteristics. We clarify the origin of steep dark current characteristics. It is confirmed that the effect of the emitter contact substantially weakens with increasing number of QWs in the QWIP structure.

In this work, we have investigated magneto-transport and polarization resolved photoluminescence of a GaAs/AlGaAs resonant tunneling diode with Si delta-doping at the center of the quantum well under a magnetic field parallel to the tunnel current. Three resonant peaks were observed in the current-voltage characteristics curve (J(V)) which were associated to donor-assisted resonant tunneling, electron resonant tunneling and to phonon-­assisted resonant tunneling. The optical emission from GaAs contact layers shows evidence of highly spin-polarized two-dimensional electron and hole gases which affect the spin-polarization of carriers in the well. The quantum well photoluminescence shows strong circular polarization degrees with values up to 85% under 15T at the donor assisted resonant tunneling peak voltage. Our results can be exploited for future development of voltage-controlled spintronics devices.

Role of electric and magnetic field on the localization of excitons in modulation-doped InGaAs/GaAs quantum well (QW) is investigated by temperature and magnetic field dependent photoluminescence (PL) spectroscopy. In this sample, an asymmetric doping profile on the one side of QW results in a strong built-in electric field along the growth direction of the structure. The carriers in QW drift according to this electric field, and experience the atomic irregularities at the InGaAs/GaAs interfaces. Under this condition, impact of thermal energy on the localization of charge carriers are probed by the temperature dependent PL measurements. On the contrary, a magnetic field along the growth direction confines the in-plane motion of charge carriers, which enhances the radiative recombination efficiency of excitons. It is found that a magnetic field with different orientations, either perpendicular or parallel to the QW plane, can be used to compensate the internal electric field assisted localization of excitons at the hetero-junctions. Moreover, a feeble Landau- diamagnetic shift of excitons under a parallel magnetic field configuration is explained by the coupling of internal electric field with the magnetic perturbation on the carriers in QW.

The band gap in photonic materials with periodic spatial modulation of refractive index greater than unity can actually be regarded as a potential barrier for photons. Similar to semiconductor quantum well systems due to the electronic band-gap mismatch, a photonic quantum well can be constructed by sandwiching a uniform medium between two photonic barriers. The transmission and reflection coefficients of light through the photonic quantum well are calculated by a modal expansion method with an R-matrix propagation algorithm. Resonance tunneling through the photonic quantum well structure is observed by varying either the well width or the frequency of incident light. Resonance peaks are found within the band-gap region, and indicate the existence of photon virtual states in the well.

Part 1 Fundamentals: blackbody radiation and flux transfer blackbody radiation flux transfer signal and noise in photoconductors optical signal of a photoconductor noise in a photoconductor. Part 2 Energy levels in one-dimensional quantum wells: analytical method Wentzel-Kramers-Brillouin approximation Kronig-Penney model transfer-matrix method. Part 3 Quantum wells in crystal lattices: finite in-plane momentum crystal symmetries and band structures of GaAs basic properties of a group symmetry group of a GaAs crystal lattice k.p perturbation theory band nonparabolicity. Part 4 Quantum wells in the presence of interaction: direct Coulomb interaction electron-electron interaction electron-phonon interaction static electric field oscillating electric field. Part 5 Intersubband transitions in quantum wells: theory of dipole transitions bound-to-bound intersubband transitions square quantum wells stepped quantum wells strongly-coupled quantum well bound-to-extended intersubband transitions bound-to-miniband transitions decoupled quantum wells coupled quantum wells. Part 6 Optical coupling for normal incidence: square gratings random gratings corrugated QWIP structures quantum grid infrared photodetectors. Part 7 Hot-electron transistors: phase coherence of ballistic electrons plasmon and phonon emission by hot-electrons magnetophonon scattering. Part 8 Excitation hot-electron spectroscopy of quantum wells: thermal excitation optical excitation. Part 9 Transport properties of multiple quantum well structures: perturbation approach to tunnelling sequential tunnelling thermionic emission thermally assisted tunnelling tunnelling in the presence of a magnetic field. Part 10 Quantum well infrared photodetectors: photoconductivity of multiple quantum wells responsivity of thin barrier MQWs electron-electron inelastic scattering in MQWs avalanche multiplication in MQWs responsivity of thick barrier MQWs capture probability quantum noise characteristics of QWIPs optimization of QWIPs. Part 11 Infrared hot-electron transistors: detector figures of merit readout circuits and dark current requirements infrared hot-electron transistors high detectively IHETs IHETs for small NEAT IHETs for high temperature operation long wavelength IHET signal amplification and quasi-photovoltaic operation of IHETs. Part 12 Noise in infrared hot-electron transistors: quantum partition noise l/f noise. Part 13 Multi-colour infrared detection: strongly-coupled asymmetric quantum wells photoexcited coherent tunnelling voltage tunable multi-colour QWIPs voltage tunable multi-colour IHETs. Part 13 Performance assessment of quantum well infrared photodetectors: an assessment of the conventional technologies an assessment of the quantum well technology. Part 14 QWIPs and IHETs in focal plane array applications: examples for QWIPs examples for IHETs.

A further development is described of an analytical approach to the problem of an impurity electron (or hole) in a quantum well (QW) subject to electric and strong magnetic external fields both directed perpendicular to the hetero-planes. Previous results for an impurity centre at the edge of a QW are extended to the case in which the impurity is located at any position in the QW. It is shown that, as in the edge case, the combined potential acting on the electron (or hole) resembles that of a double quantum well. One effective well is formed by the Coulomb potential and the QW boundary closest to the impurity. A second effective well is formed by the electric field potential and the other boundary of the QW. Analytical expressions for the energy levels of the impurity are obtained. When the levels associated with the two effective QWs anti-cross, the impurity single QW can be treated as a resonant structure. The explicit dependencies of the resonant splitting upon the width of the QW, the magnitudes of the electric and magnetic fields and the position of the impurity centre are obtained. Estimates of the expected splitting and frequency of the emitted radiation relevant to the inter-well oscillations of the electron are made using typical values associated with GaAs QWs.

We review our comprehensive investigations of electron and hole tunneling times between quantum wells separated by a single barrier. These studies show strong electronic and phonon resonances, strong influence of collisions and relaxation on tunneling times and provide new insights into nonresonant and resonant tunneling in semiconductor microstructures.

Resonant and non-resonant tunneling in [formula omitted] multi quantum well structures

The effect of the spatially inhomogeneous electric field on the quantum thermodynamic property of the particle confined in a quantum well has been investigated theoretically. The Schrodinger equation was solved for the particle subjected to a spatial confinement by an inhomogeneous electric field and energy eigen-values were obtained. Using the energy eigen-values, we have calculated the average energy, free energy, entropy, and heat capacity of this system. As an important parameter in the inhomogeneous electric field, the inhomogeneity gradient of the field plays an important role. Compared with the spatially homogenous electric field, some novel quantum effect appears. The energy eigen-value for the particle corresponding to each bound state increases as the inhomogeneity gradient increases, resulting in the increase of the average energy, free energy, and heat capacity; however, the entropy, which measures the disorder of the particle confined in the inhomogeneous field, gradually decreases with the increase of the inhomogeneity gradient. In addition, the confinement effect of the quantum well on the thermodynamic properties of this system has also investigated in great detail. The results show that the larger size of the quantum well is, the stronger the influence of inhomogeneous field becomes, which makes the thermodynamic properties of the particle in the spatially inhomogeneous field quite different from those in the homogeneous field. Our work may provide some references for the future research on the thermodynamic property of the confined atom and molecule in the inhomogeneous field and has some practical applications in statistical physics, chemical physics, and condensed physics, etc.

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In0.07Ga0.93N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schrödinger–Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.

Processes occurring when a static transverse electric field is applied to a GaAs/AlGaAs n-i-n heterostructure with single quantum wells and asymmetric tunnel-coupled double quantum wells have been investigated by optical methods. The difference between the energies of exciton transitions for quantum wells of different widths makes it possible to attribute the observed photoluminescence peaks to particular pairs of wells or particular single quantum wells. The local electric field for each quantum well has been determined in terms of the Stark shift and splitting of exciton lines in a wide range of external voltage. A qualitative model has been proposed to explain the nonmonotonic distribution of the electric field over the depth of the heterostructure.